zone loader can't find SOA record

I just built Knot 1.6.3 on MacOSX 10.10.3, and tried to run it, slaving a zone. On the first run, Knot loaded the zone from the master. At exit, it wrote the zone. However, on the second run, it logged this error:

2015-04-19T20:29:23 error: [arpa] zone loader, no SOA record, file '/Users/anandb/Homebrew/var/knot/arpa.zone'
2015-04-19T20:29:23 error: [arpa] zone reload failed (failed)

The zone file on disk does have a SOA record, and I can't see anything wrong with the zone. This doesn't appear to be a problem on Linux, so I guess it's something specific to OSX.

Knot was configured with these options:

--disable-debug --disable-dependency-tracking --disable-silent-rules --with-configdir=

--with-storage= --with-rundir= --prefix=